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Equine Herpes Myeloencephalopathy (EHM) Outbreak in Sauk County Raises Concerns

Reports of Equine Herpes Myeloencephalopthy (EHM) in five Wisconsin counties are causing concern to horse owners and equine veterinarians alike.  The reporting counties were Clark, Grant, Monroe, Sauk, and Vernon.

According to the Department of Agriculture, Trade and Consumer Protection (DATCP), “Horses at affected facilities tested positive for Equine herpesvirus-1 (EHV-1), a highly contagious virus that causes respiratory disease, abortion, and intermittent outbreaks of neurologic disease in horses.  Equine Herpes Myeloencephalopthy is often fatal.”  Because the disease is spread through horse-to-horse contact, horses at facilities where EHM has been diagnosed have been quarantined.

It's not uncommon for outbreaks of viruses to arise this time of year when equine events occur and horses commingle with animals from other farms.  The recent reports of EHM in Sauk County are a stark reminder of the importance of biosecurity measures in preventing the spread of infectious diseases among horses.  These measures should include:

1.      Quarantine and isolation of sick horses.  Sick horses should immediately be isolated from healthy individuals to prevent further spread of the virus. Additionally, horses returning from events or shows where they may have been exposed to the virus should undergo a quarantine period before reintegrating into the herd.

2.      Practice enhanced sanitation.  Regular cleaning and disinfection of stables, equipment, and communal areas are essential to reduce the risk of virus transmission. This includes disinfecting water buckets, grooming tools, and shared surfaces with appropriate antiviral agents.

3.      Monitor your horse’s temperature.  Regular monitoring of your horse’s temperature can help detect early signs of illness, allowing for prompt veterinary intervention and isolation of potentially infected horses.

4.      Restricted movement.  Limiting the movement of horses between facilities, campgrounds, etc., and avoiding unnecessary contact with unfamiliar horses can help prevent the introduction of the virus to new locations.  In short, if you don’t know where another horse has been, don’t allow your horse to have contact with it.

5.      Vaccination: Vaccinations are available for EHV-1 and EHV-4 which can help prevent respiratory and reproductive symptoms.  “Currently licensed vaccines are not labeled for the prevention of the neurologic form of EHV-1. However, vaccines may assist in limiting the spread of outbreaks of EHM by limiting nasal shedding of EHV-1 and dissemination of infection,” according to the American Association of Equine Practitioners.  Dr. Suzanne can provide risk-based vaccination recommendations for your horse during your spring wellness check.

If your horse has been exposed to any infectious or contagious disease, or if your horse has symptoms of a contagious respiratory infection, it must not be taken to shows, competitions, clinics, or public trail rides. 

If you are transporting your horse to any of these types of events, you should be aware of and seriously consider your horse’s increased risk of exposure to infectious organisms. 

While traveling with your horse, it is prudent to take a rectal temperature twice daily to help detect illness early.  If your horse has a temperature of 101 degrees F or higher, please reach out to Dr. Suzanne right away. 

When your horse returns from any type of event, you should isolate them from the rest of your herd for 21 days.  Both EVH-1 andEVH-4 spread primarily by direct or indirect contact with nasal secretions from infected horses.

For more information about practicing effective biosecurity, please visit the DATCP Division of Animal Health resources tab on their website (

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